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Hmmm. Can't wait to get home and climb up into the attic to check that out. Probably a zillion degrees up there already, but that's straight where I'm going when I get there. (current temp. in our 'burb: an even 100 at 3:30CST)

For the 4-ton unit, I would want two 20 by 25 filter racks, with a low initial clean filter, pressure drop.

Yes, that is a total of 1000-sq. ins., however, you have to figure the free air area, also it a good practice to have 350-FPM or less Velocity through the filters. That will help lower the blower's ESP, & put the velocity & static pressure where it belongs, at the Supply Air registers/diffusers for adequate throw.

Dash knows you can't load the blower wheel without having low resistance Return airflow. Both Supply air & Return air have to be properly sized so there is very little + or - differential pressure within each room. Dash knows duct layout, & he will tell the installers how to do it right.

I would put Return paths in all of the second floor rooms & keep the doors closed. I personally, would NOT put the 4-ton, 2 Return filter Racks in that very high ceiling, it will NOT work efficiently, nor properly, summer or winter!

Change the Return air situation, & do everything within your budget to reduce heat-gain! Over time those things should pay very good dividends.

My situation: I have a 1 & ½ story house (2,478 sf) with an entry and a living room that have 20’ ceilings. The second floor has half walls that look down into the living room. I think my two units are 4 ton (dn) and 2.5 ton (up). There are only two return air vents/intakes in the entire house – one for each unit – and they are both mounted in the ceiling of the upstairs hallway. Brand new coils in the big unit (old ones were leaking) and a brand new compressor in the big unit (old one died last year). Outside coils/fins are as clean.

The living room with the high ceiling faces south & west, with the west wall having a ton of windows. (By now you’re probably getting the idea this house was poorly designed?... Me too.)

Even with the thermostat set at 78, our units are running literally all day long, without a break. Granted, it’s Africa-hot here in Dallas right now, but my units couldn’t keep the house any cooler than about 82 yesterday.

My thoughts…
- We have one high arched window that is uncovered on that west wall – that’s a no-brainer that I should’ve taken care of long ago.
- Attic radiant barrier… Thinking that maybe I should study the cost & DIY possibilities of buying the rolled stuff and covering the underside of the attic.
- But, my main concern is my return air vent situation. It seems to me that this design of having both of the intakes mounted in the upstairs ceiling is a bad one. Wouldn’t the big unit be able to operate more efficiently if it were able to pull air in from the downstairs area (say, an 8’ ceiling in my dining room, for instance) ?

Any feedback or advice anyone can offer will be most appreciated.

Thanks,
Luke

You have described quite adequately what I'd refer to as the "perfect storm thermal envelope/mechanical system nightmare". I knew if I waited long enough, a fellow north Texan would be on here describing his/her woes with a house design that is completely at odds with our climate, winter and summer (especially summer). You, sir, unfortunately have been thrown into an arena of ever spiraling energy costs fused with overloaded comfort cooling systems struggling valiantly against a structural envelope design that could not be any worse at negating the chief reason a residential structure is built; to provide a sense of shelter from the elements.

But, I spare you the dramatics...; what you need is some way to tame this wild beast before the electric meter, baking in the scorching Texas sun, bankrupts you, all because you have this stubborn desire to seek respite from the heat.

For a cooling climate, high supplies and low returns are a good combination. However, a house design such as yours makes that combination a difficult feat to attain. One aspect working against you is the ductwork and air handling equipment being in the attic. If I could go back in time and find the very first instance someone thought it was smart to run cooling ducts in a space that does little more than act as a huge solar heat collector all summer day long, I'd love to bring with me reams of data, photos, etc. showing how horrible this idea is.
So...your thoughts regarding radiant barrier are thoughts in a good direction, for properly selected and applied radiant barrier can reduce heat gain to attic ducts and air handling equipment. That will give you your first boost for fighting the heat gain the rest of your structure suffers from.

I would next attack any windows that face south or west. Shade those puppies! You still have a large, exposed facade on those two orientations; some exterior shading, such as trees, arbors, etc. are worth considering. Trees admittedly have a maturity factor, so unless you buy and plant an exceptionally large tree (which is exceptionally expensive), you have to endure several seasons before that tree can help out. Arbors likely have height restrictions, and some neighborhoods with HOA's may not permit them (imagine someone telling you what you can and can't do with your own house that you make payments on...seems unAmerican to me).

By now I hope you're seeing that your problem is not just an HVAC system related one! House and mechanical systems are team members. They can either compliment one another or fight one another, with your wallet being the winner or the loser.

Your strategy for right now is twofold: ensure the mechanical systems are performing the best they possibly can. That means reducing duct leakage in the attic to an absolute minimum, ensuring all ducts flow proper amounts of air, duct penetrations from attic to interior are sealed at the ceiling plane (sometimes I wish "ceiling" was spelled "sealing", then more folks might think it should represent how it is spelled), the systems are clean, properly charged with refrigerant, etc.

Apply the radiant barrier. Insulating the roof deck would be ideal but is often cost prohibitive for many homeowners. A double whammy of sorts...one of the best solutions for reducing attic heat gain AND your electric bills ends up costing your first born and vital appendages.
Strive for as airtight of a seal between house interior and attic as you can muster.
Get some form of shading for those south and west windows and facades.

Man, you have a formidable climb ahead of you. Not insurmountable, but unfortunate. Too bad the person(s) who designed and built your house had little regard for the buyer's well being beyond getting him out of his car and into a mortgage payment. *sigh*

Well, I just can't tell you guys how happy all this good news makes me... Actually, you're all confirming what has been sinking in on me for a while.

Dash, uDarrel, Shophound and everyone else - thanks for all the great info.

I took a look at the octopus in the attic last night. Yep, just one big duct coming into the bottom - all the others in different sizes coming out of the top. That's something I'll definitely be looking into going foward - finances allowing.

Shophound paints my picture pretty accurately, I guess. I can start shooting several different directions at tackling this issue. The radiant barrier is definitely going up. Also looking into (starting today) solar screens for the back of the house. Not crazy about the look, but that wall is really too high to do anything else and the trees are too young. I guess I need to go on the warpath all over the place, too, checking for air leaks at the doors, elec. plates, etc. What about recessed lighting in the ceiling? A trouble spot, I assume... but a touchy spot, too, in terms of how to better insulate?... given the temp. issue/fire hazard?

Well, I just can't tell you guys how happy all this good news makes me... Actually, you're all confirming what has been sinking in on me for a while.

Dash, uDarrel, Shophound and everyone else - thanks for all the great info.

I took a look at the octopus in the attic last night. Yep, just one big duct coming into the bottom - all the others in different sizes coming out of the top. That's something I'll definitely be looking into going foward - finances allowing.

Shophound paints my picture pretty accurately, I guess. I can start shooting several different directions at tackling this issue. The radiant barrier is definitely going up. Also looking into (starting today) solar screens for the back of the house. Not crazy about the look, but that wall is really too high to do anything else and the trees are too young. I guess I need to go on the warpath all over the place, too, checking for air leaks at the doors, elec. plates, etc. What about recessed lighting in the ceiling? A trouble spot, I assume... but a touchy spot, too, in terms of how to better insulate?... given the temp. issue/fire hazard?

Thanks, guys. This is a great site with great folks.

Recessed lighting can be a source of sizable leakage, depending on the number and type. I remember an article titled "A Recessed Can of Worms" which you can probably look up and read. It described a case study of a house with dozens of such lights, creating serious trouble, and constructing boxes to surround them with adequate protection against overheating (size, material, I think it came out what UL would approve). If you really have trouble looking it up, you can email me with the address in the profile.

My regular HVAC pro tells me I can buy sealed replacement lights for $X at Home Depot. I have had trouble verifying such cheap pricing and regrettably I am just not that handy at DIY unless there is no other alternative.

Through another HVAC pro I bought some inserts to seal most of my canister lamps, at a cost of $XX installed. I paid up for it although I am not sure the price would be cost effective for anyone else. But that option is available, again I can tell you who did it if you want to email me.

From Home Energy Magazine Online, unfortunately now it is no longer free to access. Larry Armanda and Steve McCarthy's case study was more narrowly focused on one particular luxury home in Harrisburg PA. It dwelled on evaluating a custom solution of fabricating drywall boxes to seal the existing light fixtures. I have a hard copy of that article, did not realize everybody could not read it free of charge.

Here is some additional coverage of that subject which tends to fill in the info Armanda and McCarthy focused on:

http://www.nrel.gov/docs/fy07osti/41705.pdf (from page 35)
"ON-SITE SEALING:
==============
A site-built sealed box around the fixture (with required minimum clearances of at least 3 inches), made of drywall or other approved material, is one way to eliminate air leakage. All wiring penetrations and the joint between the box and ceiling drywall must be sealed too. It is important to note that it may be difficult or impossible to build a box that maintains minimum clearances due to the proximity of the light fixture to ceiling joists or other structural components. It is also labor-intensive to custom build these boxes and may not be cost effective depending on the number and accessibility of fixtures to the covered and the actual measured leakage rates..."

(from page 37):
'...Pacific Northwest National Laboratory’s suggestions were:
• Use only compact fluorescent bulbs or reflector type incandescents.
• Do not use any bulb that exceeds 75 watts.
• Make sure the airtight drywall boxes maintain a minimum of 3 inches of clearance to all parts of the fixture, including the terminal or junction box.
• Do not place insulation over the top of the drywall enclosure.'

So... does talk of this kind of thing, that a determined homeowner can do, fall under censure because of the DIY rules? It is certainly not the kind of work an HVAC pro would ever expect to do. I want people to be well informed but do not wish to break the rules of this site.

Too bad that article is no longer open to the public, it looked interesting. It likely reiterated in part what the one I linked to stated, but that's cool.

I would not advocate a DIY solution to leaking can lights, but rather find commercially available remedies. Too much liability otherwise, IMO. It's one thing to save a few dollars with a DIY sealing job, it's another for the structure to burn down, offsetting the benefit completely. I have no problem with DIY sealing of supply air boot to ceiling juncture, however.

"I took a look at the octopus in the attic last night. Yep, just one big duct coming into the bottom "

Same size eturn for a four ton as a two and a half ton,are you sure of the model numbers and sizes?

If and only if that's the case,they have of idea of proper design.

I will double-check and post the model numbers for you guys this evening, but I'm 99+% sure I've been told the units are 4T & 2.5T. I'm 100% certain, however, that they are visibly different in size - one larger for downstairs, and a smaller one for upstairs - and that they each have the exact same size return air grill: 20"x20". Snapped a couple of pics, too, that I'll try and post. Even took a picture of the "wall of shame" as I'd like to call it... the west-facing glass monster.